Labour was pushed into fifth place with just three per cent of the vote in the Henley by-election on Thursday. Even the Green Party and the BNP beat Labour.

And last month the party saw a 7,000 majority in Crewe and Nantwich reversed after a18 per cent swing to the Conservatives. That was the first gain the Tories had made from Labour for 30 years.

Another test of Mr Brown's popularity was the last thing the Prime Minister needed this summer.

The fact that it will be in his homeland will add to the pressure.

Last year Mr Brown suffered the embarrassment of losing the Scottish Parliament to Alex Salmond's SNP.

And the year before Labour lost a by-election in Dunfermline, where Mr Brown has his Scottish home.

The Lib Dems, having fought two poor campaigns in recent months, will also try to upset Labour in the by-election which is likely to take place later this summer.

Labour fears that the SNP will select Scottish comedienne Elaine C Smith as their candidate. She lives in the constituency and is very popular locally.

Mr Marshall has been ill for some time. It is understood that the illness may be related to depression.

However, party sources have also revealed that he felt under pressure because he had employed his wife and daughter on his Commons payroll. They add that there is no suggestion he did not abide by all the Commons rules governing expenses however.

Labour was able to claim that it put little effort into the Henley by-election because they knew it was unwinnable. However, in Glasgow they will have to launch a full blown campaign to try and hang on to the seat.